Cermet resistance composition and resistor



United States Patent 3,326,720 CERMET RESISTANCE COMPOSITION AND RESISTOR Donald A. Bruhl, Jr., Whittier, and William E. Counts, Anaheim, Calif., assignors to Beckman Instruments, Inc, a corporation of California Filed Feb. 12, 1963, Ser. No. 258,056 11 Claims. (Cl. 117--227) The present invention relates to an improved cermet electrical resistance material and elements constructed therefrom.

Cermet resistance elements presently known in the art are exemplified by U.S. Patent 2,950,995 of Thomas M. Place, Sr., et al., entitled, Electrical Resistance Element, and 2,950,996, of Thomas M. Place, Sr., et al., entitled, Electrical Resistance Material and Method of Making Same, both of which are assigned to Beckman Instruments, Inc., assignee of the present invention. These patents describe a resistance element formed by a layer of resistance material comprising a heterogenous mixture of non-conducting material and conducting metals fixed to a non-conducting base. The non-conducting material is a ceramic type material such as glass and the layer is formed by heating the metal-glass mixture at least to the melting point of the metals, so as to create a smooth, glassy phase. Additional prior art directed toward resistors formed of a glass-metal composition are U.S. Patent No. 2,837,487 of Daniel E. Huttar, entitled, Resistor Enamel and Resistor Made Therefrom, and U.S. Patent No. 2,924,540 of James B. DAndrea, entitled, Ceramic Composition and Article.

It is an object of this invention to provide a cermet resistance material for producing resistance elements having increased resistivities in proportion to the amount of metal contained therein.

Another object of this invention is to provide a cermet resistance element having a high power rating, a low noise output, and a low temperature coetficient of resistivity.

A further object of this invention is to provide an improved cermet resistance element which has extremely stable electrical characteristics, e.g., its resistivity and temperature coefficient of resistivity do not vary substantially over long periods of time, either when the element is connected in a circuit or when it is stored in a non-operative state.

Other and further objects, features and advantages of the invention will become apparent as the description proceeds.

In brief, this invention relates to the discovery that an alloy formed of the noble metals gold and iridium provides a resistance material having a very high resistivity. In addition, the gold iridium alloy may be used with at least one other noble or base metal for obtaining different ranges of resistivities and temperature coeflicients of resistivity.

A more thorough understanding of the invention may be obtained by a study of the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is an isometric view of an embodiment of the invention which is suitable for use in rotary potentiometers; and

FIG. 2 is an isometric view of another embodiment of the invention which is suitable for use in linear potentiometers as well as for fixed resistors.

In the structure of FIG. 1, a layer of resistance material is fired to a base 11, the electrodes 12, 13 being provided at each end of the layer 10 for connecting into an electrical circuit. This resistance element may be used as a fixed resistor or may be combined with a rotating contact arm for use as a rotary rheostat or potentiometer.

The base 11 may be of any suitable electrically nonconducting material which will withstand the elevated temperatures normally used to fire the resistance material. Various ceramic materials are suitable for this use, those having a smooth, fine textured surface and being impervious to moisture and other liquids being preferred. Steatite, fosterite, sintered or fused aluminas and zircon procelains are examples of preferred materials for forming the base 11.

The electrically conductive electrodes 12, 13 are conventional and may be formed by applying any of the wellknown conducting silver or other metal pastes over the layer of resistance material and firing the unit to convert the paste to a layer of metal which is firmly attached to the layer of resistance material. Alternatively, terminal structures such as are shown in the copending application of Kenneth F. Miller, et al., Ser. No. 217,689, filed Aug. 17, 1962, now Patent No. 3,134,085 entitled, Improved Terminal Structure for Resistance Elements, may be employed for making electrical contact with the resistance layer 10.

FIG. 2 illustrates another form of the resistance element of the invention in which a layer 15 of resistance material is applied to a rectangular base 16 and electrodes 17, 18 are then added at the ends of the layer 15. This form of the invention is particularly suitable for use in fixed resistors and linear otentiometers.

U.S. Patents Nos. 2,950,995 and 2,950,996 teach methods of preparing the cermet resistance layer 10. A preferred method taught therein comprises mixing the resinates of one or more noble metals. The glass binder, in the form of very small glass particles, is mixed or milled with the resinate solution so that each glass particle is thoroughly wetted with the metal solution. This mixture is gradually heated to approximately 700 F. and constantly stirred to remove the volatiles and organic materials from the mixture and to decompose the noble metal compounds. The resulting dry material is ground to a fine powder and calcined at about 850 F. The resulting calcine is ground to a fine powder, producing a dry material consisting of very small glass particles having coated thereon an extremely thin layer of minute metal particles.

The particular range of proportions of glass to metal by weight in the final resistance material may be selectively varied in the procedure just described by varying the amount of glass added to a given resinate solution. Each of the individual resinate solutions contains a predetermined quantity by weight of metal. After heating the glass and metal resinate solution, only the glass and metal remain, the amount of metal particles coated onto the glass particles being that which was originally in the resinate solutions.

The mixtures formed by the method described above may be stored indefinitely and may be used in small portions to produce limited numbers of resistance elements. When it is desired to make resistance elements using the material, the dry powder is mixed with a suitable liquid carrier to form a fluid composition which can be applied to the base material. The base with the layer applied thereto is then fired to producea continuous phase of solidified glass.

In connection with the above method, reference is made to the copending application of Ronald C. Vickery, Ser. No. 257,957, entitled, Improvements in Electrical Resistance Material and Method for Making Same," filed on even date herewith and assigned to Beckman Instruments, Inc., assignce of the present invention. This application describes and claims improvements in the art of cermet resistance elements derived from segregating the cermet material particle size into a number of very narrow ranges. The final element is constructed by using particles selected from one or more of these narrow ranges. The resulting elements have substantially more consistent reproducibility as regards their resistivity, temperature coefiicient of resistance, noise and resolution. The Vickery application, supra, further describes an improved cermet material which is obtained by varying the composition of the glass to obtain one having a particular viscosity. The elements constructed according to the present invention may be improved in accordance with the teachings of the Vickery application.

Resistance materials of this invention comprise over 16% to about 50% by weight of an iridium-gold alloy and from about 84% to 50% glass. Although not absolutely essential to obtain a resistance device, the resistance values are more easily controlled by maintaining the proportions by weight of iridium to gold Within the ratios of about 10:1 to 3:1. The following specific examples are illustrative of the invention.

Example A(20% metal) Percent Glass 80 Iridium 15 Gold 5 The resistivity of the resistance layer formed from this material was 9,130 ohms/square and its temperature coefficient of resistivity was 122 p.p.m./ C.

Example B(30% metal) Percent Glass 60 Iridium 3 0 Gold The resistivity of the resistance layer formed from this material was 1,000 ohms per square and its temperature coefficient of resistivity was +105 p.p.m./ C.

The particular composition of the glass utilized is not critical to the practice of the invention. Two illustrative examples are as follows:

The glass may be produced by any conventional process; it is preferred, however, that it be as homogeneous as possible. One method of making a glass includes thoroughly mixing a batch of raw materials together While dry, melting the batch in ceramic crucibles to produce a clear fluid glass, quenching the molten glass by pouring into cold water, drying the resulting shattered glass and then grinding it to a very fine powder.

While the glass composition is not critical, the Vickery application, supra, teaches that improved electrical characteristics may be achieved by selecting a glass composition having a predetermined viscosity. Thus, by way of example, this copending application teaches that a 20% iridium-gold, 80% glass material may be found with a particular glass composition to provide a temperature coefiicient of resistivity of 0 p.p.m./ C. The specific composition of this material is as follows:

Example C: Percent Glass A 40 Glass B 40 Iridium 17.75 Gold 2.25

The resistivity of the resistance layer formed from this material was 7,000 ohms/ square and, as noted above, its temperature coefficient of resistivity was 0 p.p.m./ C.

Certain additive metals may be included with the gold and iridium for obtaining different resistivities and temperature coefficients of resistivity. These additive metals may comprise one or more noble or base metals in the range of 0.1 mol percent to 20 mol percent of the total amount of gold and iridium. By way of example, one mol percent of the gold-iridium alloy may be replaced by a corresponding amount of palladium. This lowers the resistance of the element by approximately 40% and causes the temperature coefiicient to change in a positive manner. Silver may be added up to 16% of the total metal content for causing a decrease in resistance and a slightly more negative temperature coefficient of resistivity.

Non-noble or base type additive metals include antimony, manganese and iron. The addition of antimony lowers resistance in a manner similar to that of palladium but does not change the temperature coefficient of resistivity as much. The addition of manganese does not cause a resistance increase of the system but results in a high negative temperature coefficient of resistivity. The addition of iron causes an increase in resistance, but results in a slightly negative temperature coefiicient of resistivity.

The following example is illustrative of the addition of a base metal to the gold-iridium material.

Example D: Percent Glass 69.70 Iridium 22.5 Gold 7.5 Antimony .30

The resistivity of the resulting material was 6,000 ohms per square and its temperature coeflicient of resistivity was 69.5 p.p.m./ C.

The high resistivity of gold-iridium alloy cermet elements permits the use of a substantial amount of metal in relation to glass content without lowering the resistance of the material to an unsatisfactory value. These high metal content elements have been found to have an improved power rating and a substantially increased stability. That is, elements so constructed may be stored or used for very long periods of time, either in or out of a circuit, without substantially changing their resistivity, temperature coefficient of resistivity, noise and resolution characteristics. These resistance materials have also been found to have very good thermal stability, i.e. they may be heated and subsequently cooled without effecting a permanent change in their electrical characteristics.

Although exemplary embodiments of the invention have disclosed and discussed, it will be understood that other applications of the invention are possible and the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

We claim:

1. A resistance element comprising a high-temperature resistant, electrically nonconductive base having fired thereto a layer of resistance material consisting essentially of about 84 to 50 percent by weight of solidified glass and over 16 to about 50 percent by weight of an alloy formed of iridium and gold, said alloy being in finely divided particulate form and uniformly dispersed throughout the solidified glass.

2. A resistance element comprising a high-temperature resistant, electrically nonconductive base having fired thereto a layer of resistance material consisting essentially of about 84 to 50 percent by weight of solidified glass and over 16 to about 50 percent by weight of an alloy formed substantially of iridium and gold and a small amount of at least one other noble metal, said alloy being in finely divided particulate form and uniformly dispersed throughout the solidified glass.

3. A resistance element comprising a high-temperature resistant, electrically nonconductive base having fired thereto a layer of resistance material consisting essentially of about 84 to 50 percent by weight of solidified glass and over 16 to about 50 percent by weight of an alloy formed from iridium, gold, and a metal selected from a group consisting of palladium and silver, said alloy being in finely divided particulate form and uniformly dispersed throughout the solidified glass.

4. A resistance element comprising a high-temperature resistant, electrically nonconductive base having fired thereto a layer of resistance material consisting esentially of about 84 to 50 percent by weight of solidified glass and over 16 to about 50 percent by weight of an alloy formed substantially of iridium, gold and a metal selected from a group consisting of antimony, manganese, and iron, said alloy being in finely divided particulate form and uniformly dispersed through out the solidified glass.

5. A resistance element comprising a high-temperature resistant, electrically nonconductive base having fired thereto a layer of resistance material consisting essentially of about 84 to 50 percent by weight of solidified glass and over 16 to about 50 percent by weight of an alloy comprising gold and iridium, the proportions of iridium and gold being within the ratios of :1 to 3:1, said alloy being in finely divided form and uniformly dispersed throughout the solidified glass.

6. A resistance element comprising a high-temperature resistant, electrically nonconductive base having fired thereto a layer of resistance material consisting essentially of about 84 to 50 percent by weight of solidified glass and over 16 to about 50 percent by weight of an alloy in finely divided particulate form dispersed uniformly throughout the solidified glass, said alloy comprising gold, iridium, and a base metal selected from a group consisting of antimony, manganese, and iron, the proportions of iridium and gold by weight being within the range of ratios of 3:1 to 8:1 and the amount of said other metal being within the range of 0.10 mol percent to 20 mol percent of the total amount of gold and iridium.

7. A resistance material for use in a resistor or the like, which consists essentially of about 84 to 50 percent by weight of glass in finely divided particulate form and over 16 to about 50 percent by weight of an alloy formed of substantially iridium and gold in finely divided particulate form mixed uniformly with the glass particles.

8. A resistance material for use in a resistor or the like, which consists essentially of about 84 to percent by weight of glass in finely divided particulate form and over 16 to about 50 percent by weight of an alloy formed substantially of iridium and gold in finely divided particulate form mixed uniformly With said particles of glass, the proportions of iridium and gold by weight being within the ratios of 10:1 to 3:1. 9. A resistance material for use in a resistor or the like which consists essentially of about 84 to 50 percent by weight of glass in finely divided particulate form and over 16 to about 50 percent by weight of an alloy comprising gold, iridium, and a metal selected from the group of base metals consisting of antimony, manganese, and iron, said alloy being in finely divided particulate form and mixed with said particles of glass, the proportions of iridium and gold being within the ratios of 10:1 to 3:1 and the amount of said base metal being within the range of 0.1 mol percent to 20 mol percent of the total amount of iridium and gold. 10. A resistance material for use in a resistor or the like which consists essentially of about 84 to 50 percent by Weight glass in finely divided particulate form and over 16 to about 50 percent by weight of an alloy in finely divided particulate form mixed uniformly with said glass particles, said alloy consisting essentially of gold, iridium and at least one other noble metal, the proportions of iridium and gold being within the ratios of 10:1 to 3:1 and the amount of said other noble metal being within the range of 0.1 mol percent to 20 mol percent of the total amount of iridium and gold. 11. A resistance material for use in a resistor or the like which consists essentially of about 84 to 50 percent by weight of glass in finely divided particulate form and over 16 to about 50 percent by weight of an alloy in finely divided particulate form mixed uniformly with said particles of glass, said alloy consisting essentially of gold, iridium, and another noble metal selected from the group consisting of palladium and silver, the proportions of iridium to gold being within the ratios of 10:1 to 3:1 and the amount of said other noble metal being within the range of 0.1 mol percent to 20 mol percent of the total amount of iridium and gold.

References Cited UNITED STATES PATENTS 8/1960 Place et al. 117227 8/1965 Place et al 117--227 

1. A RESISTANCE ELEMENT COMPRISING A HIGH-TEMPERATURE RESISTANT, ELECTRICALLY NONCONDUCTIVE BASE HAVING FIRED THERETO A LAYER OF RESISTANCE MATERIAL CONSISTING ESSENTIALLY OF ABOUT 84 TO 50 PERCENT BY WEIGHT OF SOLIDIFIED GLASS AND OVER 16 TO ABOUT 50 PERCENT BY WEIGHT OF AN ALLOY FORMED OF IRIDIUM AND GOLD, SAID ALLOY BEING IN FINELY DIVIDED PARTICULATE FORM AND UNIFORMLY DISPERSED THROUGHOUT THE SOLIDIFIED GLASS. 